This is how we did our cruise last Jan and it's how I will do mine this coming January.. Wasn't hard at all, just went to the pursers desk and told them I wanted to put cash on our sail and sign card and I did not put $400.00 on my card, it was half that and then we even got money back..There were 4 of us and we all did cash.. Hope this helps...

Heidi,
You can use a credit card, in fact the majority of people do use a credit card to back their sign and sail card. They only charge the credit card once at the end of the cruise so if you want to can pay down , or off, the balance in cash towards the end so not such a large charge hits the card.

If you are going to use cash remember they put the tips on the s&s card right away so make sure to have a little cushion or you will be requested to make another deposit almost immediately if you only put down the minimum.

They take travelers checks too if you don't like having large amounts of cash on you.

From the Carnival brochure.
For your convenience, charges for most of your onboard purchases are made to you sail & sign acount. A required application form will be inclded with your documents. Carnival accepts American Express, VISA, MasterCard, Discover Card and cash for deposit on your account. (a minimum deposit will be required for cash accounts: 2 to 4 day crusies, $100; 5 to 8 day cruises $200; 9 day ao longer cruises $350)
That would be per person.

Just want to mention that if you want to pay your gratuities in cash, you should tell the purser at the beginning of your cruise so that the standard tip amount is not placed on your sign and sail card. I imagine that you can also tell them to hold off on charging the tips against your cash deposit (If you don't use a credit card as collarteral for your sign and sail charges) as well, but you might want to check on this.